Stainless steel of the &#34;20&#34; type



United States Patent STAINLESS STEEL OF THE 20 TYPE Norman S. Mott,Westfield, N. J., assignor to Cooper Alloy Corporation, Hillside, N. J.,a corporation of New Jersey No Drawing. Application April 11, 1955,Serial No. 500,693

14 Claims. (Cl. 75125) This invention relates to stainless steel, andmore particularly to stainless steel of the 20 type.

A most popular stainless steel alloy with greater corrosion resistancethan either the 304 or 316 types of the American Iron and SteelInstitute (AISI) is a high chromiuni-nickel-copper-molybdenum alloygenerally known as the 20 alloy. This has the approximate analysis 20%chromium, 28% nickel, 3.5% copper and 3% molybdenum.

This alloy is in great demand and is extensively used in the chemicaland associated industries where corrosive conditions are more exacting,and where suflicient corrosion resistance cannot be had by using suchchromiumnickel and chromium-nickel-molybdenum types as AISI 304 and 316.Unfortunately this 20 alloy has a relatively low hardness, even lowerthan the 304 and 316 types, and this low hardness has produced extremecases of galling. This precludes its successful use in corrosionresisting valves, because successive opening and closing has resulted insevere tearing and scoring of the mating seat and disc surfaces. also inother applications such as bearings where surfaces are required to slideunder load.

In industry it is an accepted fact that any tendency toward galling instainless steel may be reduced to a point where mating surfaces willsuccessfully operate without galling if there is a difference of 50 BHNor more in hardness between the mating surfaces. Such an increase of 50BHN in hardness is considered just passing, and.a greater hardnessincrease, such as 75 to 100 BHN more surely guarantees successfuloperation without galling.

The primary object of this invention is to overcome the difficulty ofgalling and seizing in the 20 type alloy by increasing the hardness ofone of the mating parts by 50 or more points of Brinell hardness.

My new alloy of the basic 20 type analysis may be made hard enough toobviate galling difliculty through the introduction of suitable amountsof nitrogen and columbium to the composition, and then heat treating todevelop their full hardening effect. In this condition the alloy fullymeets the usual high standards of corrosion resistance for a 20 typealloy, and may be used in contact with the original or base 20 metalalloy without galling.

My new alloy is an alloy of the 20 type which has been hardened by theaddition of .06 to .25 nitrogen, and also columbium in an amount eightor more times the carbon content, but not to exceed 1%, specifically, ina range of .20 to 1% (or .20 to 1.35% of columbium plus tantalumcombined), and by-further heat treating Such galling has resultedPatented June 12, 1956 the alloy in a so-called stabilizing temperaturerange, namely 1550 to 1650 F. for two to four or more hours, preferablyabout four hours, until maximum hardness is produced.

The heat hardening step follows a water quenching operation whichpreferably consists of heating the work at a temperature of from 1950 to2050 F. (preferably 2000 F.) for a period of about one hour per inchthickness, after reaching temperature, and then water quenching.

Before giving examples of my improved alloy, it may be well topreliminarily show the effect of the addition of either nitrogen aloneor columbium alone. For this purpose reference may be made to thefollowing table:

Table I Test Chegleal Analysis:

H Increase over 131 +21 +98 +39 In the foregoing and succeeding tableBI-IN means Brinell hardness number; WQ means water quenched afterheating for one hour at 2000 F.; and H means hardened by heating at 1550F. for four hours, followed by air cooling.

Example #1 shows the presence of about the maximum nitrogen contentfound in a commercial arc-melted heat. Columbium is not present in thisalloy. The BHN number of 131 may be considered to be the basic waterquench hardness. The heat treatment in the stabilizing temperature rangehas increased the Brinell hardness by 21 points. However, this alloy isnot satisfactory for heat hardening because the alloy will corrodeseverely after being given the hardening heat treatment.

Example #2 is another alloy without columbium, and in this alloy thenitrogen content has been greatly increased. It will be noted that theheat treatment has correspondingly greatly increased the BHN number to avalue of 229, or an increase of 98 over the basic number of 131. Thisexample shows that increasing the nitrogen content increases thehardness, especially after heat treatment. However, here again it isfound that the alloy is subject to severe corrosion after being giventhe heat treatment.

In Example #3 a substantial quantity of columbium has been added, butnitrogen has not been added, and the nitrogen content is small. Therewas no increase in hardness compared to Example #1 after waterquenching. There was some but not sufficient increase in hardness afterthe hardening heat treatment. However, this alloy wascorrosion-resistant even afterthe hardening heat treatment.

The eifect of increasing the nitrogen content in alloys which also havea substantial amount of columbium is illustrated in the following fourexamples of my invention:

In Example #4 the nitrogen content has been increased to 0.06 and itwill be seen that the hardness after heat treatment has been raised by56 points over the hardness number of 131 in Examples #1 and #3. Thiswould be adequate to prevent galling.

in Example #5 the nitrogen in the columbium bearing alloy has beenraised to 0.097 and the EHN has increased by 76 points over the hardnessnumber of .131 in Examples #1 and #3.

in alloy #6 the nitrogen content has been increased to 0.129 and the BHNhas increased by 98 points over the hardness number 131 in Examples #1and #3.

In Example #7 the nitrogen content has been increased to 0.25 and theBHN has been raised by 124 points over the hardness number of 131 inExamples #1 and #3.

Alloy #7 represents the maximum amount of nitrogen that it would befeasible to add, for any higher nitrogen content results in metal whichis porous because of gas holes. As a matter of fact the nitrogen contentof 0.129 in Example #6 constitutes a more desirable value for nitrogen,and the hardness increase of 98 points is more than adequate to preventgalling.

In the case of a valve. the valve body member would ordinarily be madeof the softer metal, specifically an alloy such as the #1 alloy, with nocolumbium, and solution annealed by water quenching only. With suchtreatment the alloy has the excellent corrosion resistance for which itis tnown. The valve seat disc would be made of the harder metal of my inention, including columbium and increased nitrogen, and subjected to theheat hardening treatment as Well as a preceding water quenchingtreatment. The heat hardening may be performed in a range of, say, 1550to 1650 F, the lower part of the range, say 1550 F, being preferred.

In the case of a shaft and bearing the shaft would ordinarily be made ofthe softer metal, that is a conventional #20 alloy such as Example #1above, subjected to water quenching but not heat hardening, and thebearing would be made of my new alloy, including columbium and increasednitrogen, and subjected to heat hardening after water qucnchin g.

It will be understood that an ordinary #20 alloy does not includecolumbium. However, columbium has been added in a #20 type alloy toobviate inter-granular corrosion following a welding operation. '1" bus,in situations in which it was anticipated that welding would have to beperformed, it was known to be desirable to add columbium. However, inthese alloys there was no increase in nitrogen content.

In the usual manufactured condition the 20 type alloy may have anitrogen content of from 0.01 to 0.05%. The small nitrogen content was anatural or inevitable consequence of the normal melting procedure.However, so far as I am aware, neither columbium nor nitrogen have beenadded to increase hardness. A high nitrogen content has not existed inthe alloy, and could result only after a deliberate and purposefuladdition of a high nitrogen bearing fcrro-alloy, which has not been doneheretofore.

The alloy here disclosed is primarily a casting alloy. The AISI does nothave a specified or standardized analysis for the #20 alloy, but theAlloy Casting institute (AC1) does. In this analysis the carbon is to be0.07 maximum; the chromium is to be 18 to 22%; and the nickel 21 to 31%.Other elements, specifically silicon, molybdenum, copper and manganese,are stated to vary according to proprietary formulas. in my new alloy 1recommend molybdenum to range from 2 to 4%, and copper to range from 3to 4%, these being the ranges found in the trade alloys FA-20, Carpenter20, Durimet 20, etc.

I consider silicon and manganese to be optional, with the silicon tohave a maximum limit of 2%, and the manganese to have a maximum limit of2 /2 These elements ordinarily are controlled by other factors. Forexample, the silicon content may be at the higher end of the 0 to 2%range when casting small castings, compared to large castings. Inaccordance with my invention the foregoing analysis for a #20 alloy hasadded thereto columbium in an amount at least eight times the carboncontent, but not to exceed 1%, and nitrogen in an amount ranging from0.06% to 0.25%.

It is believed that the composition and behavior of my improved heathardenablc high corrosion resistance stain- .css steel alloy, as well asthe advantages thereof, will be apparent from the foregoing detaileddescription.

Examples #1 to #7 inclusive illustrate the effect of additions ofnitrogen and columbium to an alloy having the 20 type of analysis. Withthat analysis the basic hardness is close to 131 BHN, after waterquenching, with or without columbium. The 131 BHN hardness is obtainedin the water quenched condition after heating for a period of one hourper inch, at 2000 F.

The heating may be in a temperature range from 1950 to 2050 F., with apreferred temperature of 2000" F. The period of one hour per inch meansafter reaching temperature, and usually is a one hour minimum even ifthe Work being treated is less than one inch in thickness. Usually thework is non-uniform in thickness, and the timing is adjusted to thegreater thickness. The water quenched condition is the condition for thebest corrosion resistance when columbium is not present, as in Example#1. When columbium is present in the correct amount, as in Example #3,the alloy may be subjected to a stabiliL- ing heat hardening treatment,such as heating for two to four hours at 1550 to 1650 F, followed by airor furnace cooling. This heat hardening may follow the water quenchingoperation, and the desired corrosion resistance is maintainedsuccessfully. In Example #3 the hardness is increased, but not enough toprevent galiing.

In Examples #4, #5, #6 and #7 both nitrogen and columbium are present,with the nitrogen increased beyond its normal content by adding a highnitrogen ferrochromc material to the melt. This, combined with heathardening by a stabilizing heat treatment, raises the harduessmore thanenough to prevent galling. One valve or machine part is made of astandard 20 alloy, water quenched. The other rubbing part is made of mynew alloy, heat hardened.

it will be apparent that while I have set forth specific examples of myimproved alloy, changes may be made, without departing from the scope ofthe invention, as sought to be defined in the following claims.

I claim:

1. A heat hardenable high corrosion resistance alloy, said alloyconsisting essentially of a chromium-nickelcopper-molybdenum stainlesssteel of the type known commcrcially as the #20 type (having anapproximate analysis of 20% chromium, 28% nickel, 3.5% copper, and 3%molybdenum), said steel having added thereto columbium and nitrogen, thecolumbium ranging from 0.20 to 1.00%, and the nitrogen ranging from 0.06to 0.25%.

2. A heat hardenable hi h corrosion resistance alloy, said alloyconsisting essentially of a chromium-nickelcopper-molybdenum stainlesssteel of the type known commercially as the #20 type (having anapproximate analysis of 20% chromium, 28% nickel, 3.5% copper,

and 3% molybdenum), said steel having added thereto columbium andnitrogen, the columbium ranging from 0.20 to 1.00%, and the nitrogenranging from 0.06 to 0.25%, said alloy being hardenable while retaininghigh corrosion resistance by a hardening operation consisting of heatingat about 2000" F., water quenching, and then heat hardening at about1550 F.

3. A heat hardenable alloy of high corrosion resistance, said alloyconsisting essentially of a chromium-nickelcopper-molybdenum alloy ofthe type known commercially as the type (having an approximate analysisof 20% chromium, 28% nickel, 3.5 copper, and 3% molybdenum), said steelhaving columbium in an amount at least eight times the carbon contentand not over 1%, and having nitrogen in a range of from 0.06% to 0.25%.

4. A heat hardenable alloy of high corrosion resistance, said alloyconsisting essentially of a chromium-nickelcopper-molybdenum alloy ofthe type known commercially as the #20 type (having an approximateanalysis of 20% chromium, 28% nickel, 3.5 copper, and 3% molybdenum),said steel having columbium in a range of from 0.2 to 1% and havingnitrogen in a range of from 0.06% to 0.25 and the carbon content beingless than one-eighth the columbium content.

5. A heat hardenable alloy of high corrosion resistance, said alloyconsisting essentially of a chromium-nickelcopper-molybdenum alloy ofthe type known commercially as the #20 type (having an approximateanalysis of 20% chromium, 28% nickel, 3.5% copper, and 3% molybdenum),said steel having columbium in a range of from 0.2 to 1% and havingnitrogen in a range of from 0.06% to 0.25%, the columbium content beingat least eight times the carbon content, and said alloy being heathardenable to a BHN hardness at least fifty points higher than aconventional #20 type alloy while retaining high corrosion resistance,so that the two alloys may be used in rubbing engagement withoutgalling.

6. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having about 20% chromium, 28% nickel, 3.5%copper, 3% molybdenum, 0.20 to 1.00% columbium, 0.06 to 0.25% nitrogen,the remainder being essentially iron with other ingredients common tothe #20 type alloy.

7. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having a range of from about 18 to 22%chromium, 21 to 31% nickel, 0 to 2% silicon, 0 to 2%% manganese, 3 to 4%copper, 2 to 4% molybdenum, 0.20 to 1.00% columbium, 0.06 to 0.25%nitrogen, the remainder being essentially iron with a carbon content notexceeding about 0.07%.

8. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having a range of from about 18 to 2.2%chromium, 21 to 31% nickel, 0 to 2% silicon, 0 to 2 /t% manganese, 3 to4% copper, 3 to 4% molybdenum, 0.06 to 0.25 nitrogen, carbon in anamount not exceeding about 0.07%, and columbium in an amount at leasteight times the carbon but not exceeding 1%, the remainder beingessentially iron.

9. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having approximately the following chemicalanalysis: carbon 0.039%, chromium 20.10%, nickle 28.30%, silicon 1.81%,manganese 0.47%, copper 3.00%, molybdenum 3.63%, columbium 0.79%,nitrogen 0.06%, and the balance of the alloy being substantially alliron.

10. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having approximately the following chemicalanalysis: carbon 0.053%, chromium 19.93%, nickel 28.60%, silicon 1.65%,manganese 0.68%, copper 3.04%, molybdenum 3.80%, columbium 0.77%,nitrogen 0.097%, and the balance of the alloy being substantially alliron.

11. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having approximately the following chemicalanalysis: carbon 0.057%, chromium 20.15%, nickel 28.10%, silicon 1.96%,manganese 0.64%, copper 3.02%, molybdenum 3.63%, columbium 0.79%,nitrogen 0.129%, and the balance of the alloy being substantialy alliron.

12. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having approximately the following chemicalanalysis: carbon 0.057%, chromium 20.15%, nickel 28.10%, silicon 1.96%,manganese 0.64%, copper 3.02%, molybdenum 3.63%, columbium 0.79%,nitrogen 0.129%, and the balance of the alloy being substantially alliron, the said alloy being adapted to be heat hardened to a BHN at leastfifty points greater than that of a conventional #20 type alloy whileretaining its high corrosion resistance.

13. A heat hardenable high corrosion resistance stainless steel alloy ofthe #20 type, said alloy having approximately the following chemicalanalysis: carbon 0.058%, chromium 19.75%, nickel 27.90%, silicon 1.92%,manganese 0.57%, copper 3.15%, molybdenum 3.95%, columbium 0.80%,nitrogen 0.250%, and the balance of the alloy being substantially alliron.

14. Two parts both made of high corrosion resistance stainless steel ofthe type known commercially as the #20 type (having an approximateanalysis of 20% chromium, 28% nickel, 3.5% copper, and 3% molybdenum),said parts being used in combination, in sliding contact under pressure,one of said parts being the usual #20 alloy, solution annealed byheating and water quench-ing, and the other of said parts being made ofan alloy of the #20 type having added thereto columbium and nitrogen,the columbinum ranging from 0.20 to 1.00%, and the nitrogen ranging from0.06 to 0.25%, hardened by heating, water quenching, and heat hardening,there being a difference in BHN hardness of at least fifty points, toprevent galling.

References Cited in the file of this patent UNITED STATES PATENTS

1. A HEAT HARDENABLE HIGH CORROSION RESISTANCE ALLOY, SAID ALLOYCONSISTING ESSENTIALLY OF A CHROMIUM-NICKELCOPPER-MOLYBDENUM STAINLESSSTEEL OF THE TYPE KNOWN COMMERCIALLY AS THE #20 TYPE (HAVING ANAPPROXIMATE ANALYSIS OF 20% CHROMIUM, 28% NICKEL, 3,5% COPPER, AND 3%MOLYBDENUM), SAID STEEL HAVING ADDED THERETO COLUMBIUM AND NITROGEN, THECOLUMBIUM RANGING FROM 0.20 TO 1.00%, AND THE NITROGEN RANGING FROM 0.06TO 0.25%.